If you spend any time in the woods, whether it be hunting or hiking or whatever, you might want to pay attention to the trees. If you see purple paint on the trees while you're in the woods, you're going to want to turn around and head out. Although it's not a law in NYS as of yet, the idea is to help property owners mark their land without having to put "No Trespassing" signs up. The paint is easier to maintain for property owners and can't be hidden or tampered with.
The purple paint is to signify that you are on someone's property that doesn't want you there.